linux-kernel - Re: [RFC] CMA page migration failure due to buffers on bh

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Date:	Tue, 1 Jul 2014 22:46:21 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Laura Abbott <lauraa@...eaurora.org>
Cc:	Gioh Kim <gioh.kim@....com>, Michal Nazarewicz <mina86@...a86.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Mel Gorman <mgorman@...e.de>,
	이건호 <gunho.lee@....com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC] CMA page migration failure due to buffers on bh_lru

On Mon, 30 Jun 2014 19:02:45 -0700 Laura Abbott <lauraa@...eaurora.org> wrote:

> On 6/30/2014 6:07 PM, Gioh Kim wrote:
> > Hi,Laura.
> > 
> > I have a question.
> > 
> > Does the __evict_bh_lru() not need bh_lru_lock()?
> > The get_cpu_var() has already preenpt_disable() and can prevent other thread.
> > But get_cpu_var cannot prevent IRQ context such like page-fault.
> > I think if a page-fault occured and a file is read in IRQ context it can change cpu-lru.
> > 
> > Is my concern correct?
> > 
> > 
> 
> __evict_bh_lru is called via on_each_cpu_cond which I believe will disable irqs.
> I based the code on the existing invalidate_bh_lru which did not take the bh_lru_lock
> either. It's possible I missed something though.

I fear that running on_each_cpu() within try_to_free_buffers() is going
to be horridly expensive in some cases.

Maybe CMA can use invalidate_bh_lrus() to shoot down everything before
trying the allocation attempt.  That should increase the success rate
greatly and doesn't burden page reclaim.  The bh LRU isn't terribly
important from a performance point of view, so emptying it occasionally
won't hurt.

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